Phase-change medium usable in phase-difference tracking

ABSTRACT

A product for recording information by use of laser light includes a recording layer being able to change between an amorphous state and a crystalline state so as to record information, the recording layer having a thickness falling within a first range defined relative to a wavelength of the laser light, a protection layer having a thickness falling within a second range defined relative to the wavelength of the laser light, a heat releasing layer having a thickness falling within a third range defined relative to the wavelength of the laser light, and a transparent substrate supporting the layers. The first, second, and third ranges are selected such that a phase of the laser light reflected by the amorphous state and a phase of the laser light reflected by the crystalline state have a phase difference larger than a predetermined phase amount.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical phase-change recordingmedium which records and erases information by changing a phase of arecording layer between an amorphous state and a crystalline state.

2. Description of the Related Art

A DVD-RAM, which permits recording, reproduction, and erasure ofinformation, has been attracting wide attention as it has advantagesover a DVD-ROM, which is only capable of reproduction, and a DVD-R,which is only capable of recording and reproduction.

In regard to an optical-disk drive, a scheme that obtains atracking-error signal through phase-difference tracking is known in theart. Reference may be made to Japanese Patent Publication No. 56-30610and Japanese Patent Publication No. 2-56734, the contents of which arehereby incorporated by reference. In this scheme, a laser-beam spot isscanned over a recording track, which is comprised of a series ofreflection pits each formed as a lump or a recess. An optical phase ofreflected light is then modulated so as to detect a change in anopticalintensity distribution observed on a receiver device. Based onthis change, a displacement of the series of pits from an expectedcenter thereof is detected. A position of the laser-beam spot issubjected to servo control to minimize the displacement, therebyeffecting appropriate tacking.

No optical phase-change recording medium; however, is known to datewhich has a memory capacity comparable to that of the DVD-ROM and can bedriven by a DVD-ROM drive.

One of the reasons why such medium does not exist is that opticalphase-change recording media usually exhibit a low reflectivity andallow only a small degree of amplitude modulation to be obtained, asphysical and signal characteristics are concerned. Such characteristicsresult in an insufficient signal magnitude. Because of this reason, itis difficult to drive a DVD-RAM in the same manner as driving DVD-ROM ina DVD-ROM drive. Particularly, a phase-difference tracking signal, whichis necessary for tracking a series of pits, cannot be obtained in asufficient magnitude, giving rise to a problem in that the trackingcontrol becomes unstable.

Accordingly, there is a need for an optical phase-change medium whichpermits appropriate phase-difference tracking.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean optical phase-change medium which can satisfy the need describedabove.

It is another and more specific object of the present invention toprovide an optical phase-change medium which permits appropriatephase-difference tracking.

In order to achieve the above objects according to the presentinvention, a product for recording information by use of laser lightincludes a recording layer being able to change between an amorphousstate and a crystalline state so as to record information, the recordinglayer having a thickness falling within a first range defined relativeto a wavelength of the laser light, a protection layer having athickness falling within a second range defined relative to thewavelength of the laser light, a heat releasing layer having a thicknessfalling within a third range defined relative to the wavelength of thelaser light, and a transparent substrate supporting the layers. Thefirst, second, and third ranges are selected such that a phase of thelaser light reflected by the amorphous state and a phase of the laserlight reflected by the crystalline state have a phase difference largerthan a predetermined phase amount.

According to one aspect of the present invention, the phase differenceis substantially larger than 10°. The larger the phase difference, thegreater the output of phase-difference tracking becomes. As a matter offact, the phase-difference-tracking output exceeds 0.3 V when theoptical phase difference is lager than 10°. This means that an outputlevel comparable to that of the DVD-ROM is obtained even whenphase-difference tracking is employed. Namely, the optical phase-changerecording medium as described above can be played on a DVD-ROM player.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative drawing showing layers of an opticalphase-change recording medium according to an embodiment of the presentinvention;

FIG. 2 is a chart showing a relation between a phase-difference-trackingoutput magnitude and an optical phase difference of the opticalphase-change recording medium;

FIG. 3 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofa recording layer;

FIG. 4 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofa lower protection layer placed between a transparent substrate and therecording layer;

FIG. 5 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofa heat releasing layer;

FIG. 6 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofan upper protection layer;

FIG. 7 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different materials usedas the recording layer;

FIG. 8 is a table chart showing whether it was possible to achievephase-difference tracking under different conditions in which a ratio ofan amorphous-state reflectivity to a crystalline-state reflectivity waschanged;

FIG. 9 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different groove depthsof the transparent substrate;

FIG. 10 is a table chart showing phase-difference-tracking outputmagnitudes which were measured for different track pitches of theoptical phase-change recording medium;

FIG. 11 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different groove widthsof the transparent substrate; and

FIG. 12 is an illustrative drawing showing power of laser pulses used inan overwrite operation with respect to the optical phase-changerecording medium according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, a principle and embodiments of the present inventionwill be described with reference to the accompanying drawings.

FIG. 1 is an illustrative drawing showing layers of an opticalphase-change recording medium 1 according to an embodiment of thepresent invention.

As shown in the figure, the optical phase-change recording medium 1includes a transparent substrate 2 made of polycarbonate or the like andother layers formed one over another on the transparent substrate 2.These layers include a lower protection layer 3 (e.g., made of aphase-change material such as ZnS, SiO₂, or SiN_(x)), a recording layer4 (e.g., made of AgInSbTe, GeSbTe, or the like), a upper protectionlayer 5 (e.g., made of ZnS.SiO₂, SiN_(x)), a heat releasing layer 6(e.g., made of Al, Al alloy, Au, Ag, or the like), and a UV protectionlayer 7 (e.g., made of a UV-curable resin). The present invention is notlimited to this particular layer structure and particular materials.This layer structure and particular materials are preferable, however,because sufficient characteristics are achieved in terms of recording,reproduction, and erasure of information. The optical phase-changerecording medium 1 may be formed as a disk, a card, a sheet, or whatevershape that may be appropriate.

The optical phase-change recording medium 1 having a multi-layerstructure as described above has carefully selected specifications asdescribed in the following. These specifications are intended to providea phase-difference tracking signal of a sufficient magnitude comparableto that of the DVD-ROM, thereby making it possible to play the medium ona DVD-ROM player.

[1] Optical Phase Difference Larger Than 10°

FIG. 2 is a chart showing a relation between a phase-difference-trackingoutput magnitude and an optical phase difference of the opticalphase-change recording medium 1. The optical phase difference is definedas “Φ1-Φ2”, where Φ represents a phase of light reflected by anamorphous-state mark recorded in the recording layer 4, and Φ2represents a phase of light reflected by a crystalline-state portion. Asshown in FIG. 2, the phase-difference-tracking output magnitude exceeds0.3 V when the optical phase difference is lager than 10°. This meansthat an output level comparable to that of the DVD-ROM is obtained evenwhen phase-difference tracking is employed. Namely, the opticalphase-change recording medium 1 can be played on a DVD-ROM player.

[2] Thickness of Record Layer 0.027 to 0.033 Times Wavelength of LaserLight

FIG. 3 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofthe recording layer 4. Measurements shown in the table were obtainedunder such conditions as follows. The wavelength L of the laser lightwas 635 [nm], and the numerical aperture NA was 0.6 with respect to anobject lens of the optical pick-up device. The line density of the trackwas 0.3 [μm/bit]. Also, the signal format used for measurement was EFMmodulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. AgInSbTe was used for the recordinglayer 4. ZnS.SiO₂ was used for the lower protection layer 3, thethickness of which was 160 nm. The upper protection layer 5 was formedfrom ZnS.SiO₂, and had a layer thickness of 20 nm. Also, the heatreleasing layer 6 was formed from an Al alloy, and a layer thicknessthereof was 120 nm.

As shown in FIG. 3, the phase-difference-tracking output magnitude wasas large as or larger than 0.25 V when the layer thickness of therecording layer 4 was 0.027 to 0.033 times the wavelength of the laserlight. Under this condition, also, jitters were just as favorable asthey should be. In this manner, an output level as high as that of theDVD-ROM can be obtained even when phase-difference tracking is employed.The optical phase-change recording medium 1 can thus be driven by aDVD-ROM player.

[3] Thickness of Lower Protective Layer 0.20 to 0.32 Times Wavelength ofLaser Light

FIG. 4 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofthe lower protection layer 3 placed between the transparent substrate 2and the recording layer 4. Measurements shown in the table were obtainedunder such conditions as follows. The wavelength L of the laser lightwas 635 [nm], and the line density of the track was 0.3 [μm/bit]. (Thisline density is only an example, and recording of the media can besuccessful with a jitter less than 10% if the line density of the trackis greater than 2.5 [μm/bit].) Also, the signal format used formeasurement was EFM modulation.

As shown in FIG. 4, the phase-difference-tracking output magnitude wasas large as or larger than 0.20 V when the layer thickness of the lowerprotection layer 3 was 0.20 to 0.32 times the wavelength of the laserlight. Under this condition, also, jitters were just as favorable asthey should be. In this manner, an output level as high as that of theDVD-ROM can be obtained even when phase-difference tracking is employed.The optical phase-change recording medium 1 can thus be driven by aDVD-ROM player.

Here, the following is the specifics with regard to the opticalphase-change recording medium 1. ZnS.SiO₂ was used for the lowerprotection layer 3. AgInSbTe was used for the recording layer 4, thethickness of which was 17 nm. The upper protection layer 5 was formedfrom ZnS.SiO₂, and had a layer thickness of 20 nm. Also, the heatreleasing layer 6 was formed from an Al alloy, and a layer thicknessthereof was 120 nm.

[4] Thickness of Heat Releasing Layer 0.13 to 0.25 Times Wavelength ofLaser Light

FIG. 5 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofthe heat releasing layer 6. Measurements shown in the table wereobtained under such conditions as follows. The wavelength L of the laserlight was 635 [nm], and the line density of the track was 0.3 [μm/bit].Also, the signal format used for measurement was EFM modulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The heat releasing layer 6 was formedfor example from a metal layer such as an Al alloy. ZnS.SiO₂ was usedfor the lower protection layer 3, the thickness of which was 160 nm.AgInSbTe was used for the recording layer 4, and the layer thicknessthereof was 17 nm. The upper protection layer 5 was formed fromZnS.SiO₂, and had a layer thickness of 20 nm.

As shown in FIG. 5, the phase-difference-tracking output magnitude wasas large as or larger than 0.25 V, and jitters were just as favorable asthey should be when the layer thickness of the heat releasing layer 6was 0.13 to 0.25 times the wavelength of the laser light. In thismanner, an output level as high as that of the DVD-ROM can be obtainedeven when phase-difference tracking is employed. The opticalphase-change recording medium 1 can thus be driven by a DVD-ROM player.

[5] Thickness of Upper Protective Layer 0.025 to 0.038 Times Wavelengthof Laser Light

FIG. 6 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different thicknesses ofthe upper protection layer 5. Measurements shown in the table wereobtained under such conditions as follows. The wavelength L of the laserlight was 635 [nm], and the line density of the track was 0.3 [μm/bit].Also, the signal format used for measurement was EFM modulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The upper protection layer 5 was formedfrom ZnS.SiO₂. ZnS.SiO₂ was used for the lower protection layer 3, thethickness of which was 160 nm. AgInSbTe was used for the recording layer4, and the layer thickness thereof was 17 nm. The heat releasing layer 6was formed from an Al alloy, and had a layer thickness of 120 nm.

As shown in FIG. 6, the phase-difference-tracking output magnitude wasas large as or larger than 0.25 V, and jitters were just as favorable asthey should be when the layer thickness of the upper protection layer 5was 0.025 to 0.038 times the wavelength of the laser light. In thismanner, an output level as high as that of the DVD-ROM can be obtainedeven when phase-difference tracking is employed. The opticalphase-change recording medium 1 can thus be driven by a DVD-ROM player.

[6] Record Layer Including Ag, In, Sb, and Te

FIG. 7 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different materials usedas the recording layer 4. Measurements shown in the table were obtainedunder such conditions as follows. The wavelength L of the laser lightwas 635 [nm], and the line density of the track was 0.3 [μm/bit]. Also,the signal format used for measurement was EFM modulation. Recordmaterial used here was either AgInSbTe or GeSbTe.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The recording layer 4 had a thicknessthereof equal to 17 nm. The upper protection layer 5 was formed fromZnS.SiO₂, and had a layer thickness of 20 nm. The lower protection layer3 was made of ZnS.SiO₁₂, and a layer thickness thereof was 160 nm.Further, the heat releasing layer 6 was an Al alloy with a layerthickness of 120 nm.

As shown in FIG. 7, the phase-difference-tracking output magnitude wasas large as 0.25 V, and jitters were just as favorable as they should bewhen AgInSbTe was used for the recording layer 4. In this manner, anoutput level as high as that of the DVD-ROM can be obtained even whenphase-difference tracking is employed. The optical phase-changerecording medium 1 can thus be driven by a DVD-ROM player.

When a material such as GeSbTe was used, jitters undesirably exceeded10% under the employed condition of line density, i.e., 0.3 μm/bit. Inaddition, the phase-difference-tracking output magnitude was smallerwith this material. This means that appropriate tracking cannot beconducted.

[7] Reflectivity of Amorphous State Larger Than 50% of Reflectivity ofCrystalline State

FIG. 8 is a table chart showing whether it was possible to achievephase-difference tracking under different conditions in which a ratio(Ra/Rc) of amorphous-state reflectivity Ra to crystalline-statereflectivity Rc was changed. Other specific conditions were as follows.The wavelength L of the laser light was 635 [nm], and the line densityof the track was 0.3 [μm/bit]. Also, the signal format used formeasurement was EFM modulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The recording layer 4 was AgInSbTe witha layer thickness of 17 nm. The upper protection layer 5 was formed fromZnS.SiO₂, and had a layer thickness of 20 nm. The lower protection layer3 was made of ZnS.SiO₁₂, and a layer thickness thereof was 160 nm.Further, the heat releasing layer 6 was an Al alloy with a layerthickness of 120 nm.

As shown in FIG. 8, it was possible to establish phase-differencetracking when the ratio Ra/Rc was no smaller than 50%. In this manner,an output level as high as that of the DVD-ROM can be obtained even whenphase-difference tracking is employed. The optical phase-changerecording medium 1 can thus be driven by a DVD-ROM player.

[8] Transparent Substrate Having Groove Depth of 40 to 60 nm

FIG. 9 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different groove depthsof the transparent substrate 2. Measurements shown in the table wereobtained under such conditions as follows. The wavelength L of the laserlight was 635 [nm], and the line density of the track was 0.3 [μm/bit].Also, the signal format used for measurement was EFM modulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The recording layer 4 was AgInSbTe witha layer thickness of 17 nm. The upper protection layer 5 was formed fromZnS.SiO₂, and had a layer thickness of 20 nm. The lower protection layer3 was made of ZnS.Si0 ₁₂, and a layer thickness thereof was 160 nm.Further, the heat releasing layer 6 was an Al alloy with a layerthickness of 120 nm.

As shown in FIG. 7, the phase-difference-tracking output magnitude wasas large as or larger than 0.25 V, and jitters were just as favorable asthey should be when the groove depth of the transparent substrate 2ranged from 40 to 60 nm. In this manner, an output level as high as thatof the DVD-ROM can be obtained even when phase-difference tracking isemployed. The optical phase-change recording medium 1 can thus be drivenby a DVD-ROM player.

[9] Track Pitch Ranging 0.6 μm-0.8 μm

FIG. 10 is a table chart showing phase-difference-tracking outputmagnitudes which were measured for different track pitches of theoptical phase-change recording medium 1. Measurements shown in the tablewere obtained under such conditions as follows. The wavelength L of thelaser light was 635 [nm], and the line density of the track was 0.3[μm/bit]. Also, the signal format used for measurement was EFMmodulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The recording layer 4 was AgInSbTe witha layer thickness of 17 nm. The upper protection layer 5 was formed fromZnS.SiO₂, and had a layer thickness of 20 nm. The lower protection layer3 was made of ZnS.SiO₁₂, and a layer thickness thereof was 160 nm.Further, the heat releasing layer 6 was an Al alloy with a layerthickness of 120 nm.

As shown in FIG. 10, the phase-difference-tracking output magnitude wasas large as or larger than 0.25 V when the track pitch ranged from 0.6μm to 0.8 μm. In this manner, an output level as high as that of theDVD-ROM can be obtained even when phase-difference tracking is employed.The optical phase-change recording medium 1 can thus be driven by aDVD-ROM player.

[10] Transparent Substrate Having Groove Width of 0.25 to 0.40 μm

FIG. 11 is a table chart showing phase-difference-tracking outputmagnitudes and jitters which were measured for different groove widthsof the transparent substrate 2. Measurements shown in the table wereobtained under such conditions as follows. The wavelength L of the laserlight was 635 [nm], and the line density of the track was 0.3 [μm/bit].Also, the signal format used for measurement was EFM modulation.

Further, the following is the specifics with regard to the opticalphase-change recording medium 1. The recording layer 4 was AgInSbTe witha layer thickness of 17 nm. The upper protection layer 5 was formed fromZnS.SiO₂, and had a layer thickness of 20 nm. The lower protection layer3 was made of ZnS.SiO₁₂, and a layer thickness thereof was 160 nm.Further, the heat releasing layer 6 was an Al alloy with a layerthickness of 120 nm.

As shown in FIG. 11, the phase-difference-tracking output magnitude wasas large as or larger than 0.21 V, and jitters were just as favorable asthey should be when the groove width of the transparent substrate 2ranged from 0.25 μm to 0.40 μm. In this manner, an output level as highas that of the DVD-ROM can be obtained even when phase-differencetracking is employed. The optical phase-change recording medium 1 canthus be driven by a DVD-ROM player.

[11] Line Velocity for Recording and Reproducing Ranging 3 m/s-8 m/s

When the linear velocity of the optical phase-change recording medium 1is set to 3-8 m/s with respect to the recording and reproduction ofinformation, this achieves a linear velocity which is 1 to 2 times asfast as that of the conventional DVD ROM. This allows the opticalphase-change recording medium 1 to be played by a DVD-ROM player.

In what follows, specifics of the embodiment of the present inventionwill be described.

The specifics of the optical phase-change recording medium 1 are asfollows. The transparent substrate 2 is formed from polycarbonate (PC)with a refractive index of 1.58. On the transparent substrate 2, thelower protection layer 3 is formed from the ZnS.SiO₂ to have a layerthickness of 160 nm. On the lower protection layer 3, the recordinglayer 4 is provided by using Ag2In1OSb28Te6O, and has a thicknessthereof equal to 17 nm. On the recording layer 4, the upper protectionlayer 5 is formed by using ZnS.SiO₂ with a layer thickness of 20 nm anda refractive index of 2.1. The heat releasing layer 6 is formed on theupper protection layer 5. A material of the heat releasing layer 6 is anAl alloy (Al—Ti: 1 wt %), and a thickness thereof is 120 nm. On theupper protection layer 5, the UV protection layer 7 is formed as thetopmost layer based on UV-curing resin.

A reproduction device, which reproduces information from the opticalphase-change recording medium 1, is configured as follows. Thewavelength L of the laser light is 635 [nm], and the numerical apertureNA is 0.6 with respect to an object lens of the optical pick-up device.The line density of the track is 0.3 [μm/bit]. Further, the laser beamat the time of an overwriting operation has a multi-pulse beam patternwith a peak power at 12 mW, a bottom power at 5 mW, and a lead power 1mW as shown in FIG. 12. The figure shows 4T write pulses.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

The present application is based on Japanese priority applicationNo.10-013661 filed on Jan. 27, 1998, with Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

Further, the present application relates to a U.S. application Ser. No.08/807632 filed on Feb. 27, 1997, with the United States Patent andTrademark Office, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A product for recording information by use of a laser light, comprising: a recording layer configured to change between an amorphous state and a crystalline state so as to record information, said recording layer having a thickness falling within a first range defined relative to a wavelength of the laser light; a protection layer having a thickness falling within a second range defined relative to the wavelength of the laser light; a metal layer having a thickness falling within a third range defined relative to the wavelength of the laser light; and a transparent substrate supporting said recording, protection, and metal layers, wherein the first, second, and third ranges are selected such that a phase of the laser light reflected by the amorphous state and a phase of the laser light reflected by the crystalline state have a phase difference larger than a predetermined phase amount, and the first range is between 0.023 times the wavelength of the laser light and 0.033 times the wavelength of the laser light.
 2. The product as claimed in claim 1, wherein the predetermined phase amount is substantially 10°.
 3. The product as claimed in claim 1, wherein the third range is between 0.13 times the wavelength of the laser light and 0.25 times the wavelength of the laser light.
 4. The product as claimed in claim 1, wherein said protection layer is placed between said transparent substrate and said recording layer, and said second range is between 0.23 times the wavelength of the laser light and 0.28 times the wavelength of the laser light.
 5. The product as claimed in claim 1, wherein said protection layer is placed between said recording layer and said metal layer, and said second range is between 0.026 times the wavelength of the laser light and 0.036 times the wavelength of the laser light.
 6. The product as claimed in claim 1, wherein said recording layer includes Ag, In, Sb, and Te.
 7. The product as claimed in claim 1, wherein a reflectivity of the amorphous state is greater than half a reflectivity of the crystalline state with respect to the laser light.
 8. The product as claimed in claim 1, wherein said transparent substrate has grooves therein having a depth between 40 nm and 60 nm.
 9. The product as claimed in claim 1, wherein said recording layer has tracks formed thereon, and a pitch of the tracks is between 0.6 μm and 0.8 μm.
 10. The product as claimed in claim 1, wherein said transparent substrate has grooves therein having a width between 0.25 μm and 0.40 μm.
 11. A method of recording and reproducing information in and from an optical phase-change recording medium, comprising recording and reproducing information in and from the product of claim 1 at a linear velocity falling within a range between 3 m/s and 8 m/s.
 12. A method of forming an optical phase-change recording medium used for recording information by use of a laser light, comprising: forming a recording layer configured to change between an amorphous state and a crystalline state so as to record information, such that said recording layer has a thickness falling within a first range defined relative to a wavelength of the laser light; forming a protection layer having a thickness falling within a second range defined relative to the wavelength of the laser light; forming a metal layer having a thickness falling within a third range defined relative to the wavelength of the laser light; and forming a transparent substrate for supporting said recording, protection, and metal layers, wherein the first, second, and third ranges are selected such that a phase of the laser light reflected by the amorphous state and a phase of the laser light reflected by the crystalline state have a phase difference larger than a predetermined phase amount, and the first range is between 0.023 times the wavelength of the laser light and 0.033 times the wavelength of the laser light.
 13. The method as claimed in claim 12, wherein the predetermined phase amount is substantially 10°.
 14. The method as claimed in claim 12, wherein the third range is between 0.13 times the wavelength of the laser light and 0.25 times the wavelength of the laser light.
 15. The method as claimed in claim 12, wherein said protection layer is placed between said transparent substrate and said recording layer, and said second range is between 0.23 times the wavelength of the laser light and 0.28 times the wavelength of the laser light.
 16. The method as claimed in claim 12, wherein said protection layer is placed between said recording layer and said metal layer, and said second range is between 0.026 times the wavelength of the laser light and 0.036 times the wavelength of the laser light. 